Temperature control unit for piezoelectric crystal



Sept. 8, 1953 E. E. OVERBEY ETA L 2,651,731

TEMPERATURE CONTROL UNIT FOR PIEZOELECTRIC CRYSTAL Filed Feb. 2, 1951d1'romsyf Patented Sept. 8, 1953 TEMPERATURE CONTROL UNIT FOR IPIEZOELECTRIC CRYSTAL Ernest E. Overbey and John D. Holmbeck, Sandwich,Ill., assignors to The James Knights Company, nois Sandwich, 111., acorporation of Illi- Application February 2, 1951, Serial No. 209,002

cally controlled oven for maintaining a piezoelectric crystal at apredetermined temperature.

It is an object of the invention to provide an improved crystal oven formaintaining a piezoelectric crystal at a predetermined temperature witha high degree of precision in spite of large and sudden changes inthe'ambient conditions. It is another object to provide an improvedcrystal oven for use with the standard, commonly employed can type ofcrystal holder, in which a plurality of such holders, preferably two,may be accommodated.

It is a further object of the invention to provide a novel crystal ovenwhich is simple and inexpensive to construct, which is extremely compactand which, nevertheless, may be easily and quickl disassembled andserviced.

Other objects and advantages of the present construction will beapparent upon reference to the detailed description and the accompanyingdrawing in which:

Figure 1 is an exploded perspective of the crystal oven constructed inaccordancev with the present invention.

Fig. 2 shows the appearance of the crystal oven assembled and includinga protective cover.

Fig. 3 is a cross section of the crystal oven taken along the line 33 inFigure 1. V

Fig. 4 is a bottom view of the heater assembly.

Fig. 5 is a schematic diagram of the, electrical connections.

While the invention is susceptible of various modifications andalternative constructions and uses, we have shown in the drawings andwill herein describe in detail the embodiment of the invention; it is tobe understood, however, that we do not intend to limit the invention bysuch disclosure, but aim to cover all modifications and alternativeconstructions and uses falling within the scope of the invention asexpressed in the appended claims.

Referring to Figure 1, it will be seen that'the oven comprises a baseassembly l0, a heater assembly II and a cover l2. The base assembly ispreferably designed to fit in a standard octal tube socket and includesprongs 2l23. The base assembly is preferably formed of moulded phenolicor the like and includes an upraised portion or island 30 having pinsockets 3I-34 inclusive. These pin sockets are adapted to receive thecontact pins of two crystal holders 35-36 of the well known rectangularcan type. Appropriate connections are made from the pin sockets 3l34 to.the octal prongs although such connections have been omitted in thedrawing 4 Claims. (Cl. 310-9) for the sake of simplicity. The baseassembly further includes upwardly projecting posts 31, 38, 39 which areelectrically connected to prongs 2 I, 22, 23 for the purpose ofestablishing electrical contact with the heater assembly I I. I

The heater assembly II, which is dimensioned to telescope over thecrystal holders 35, 36, includes an inverted cup 40 which may be ofcopper, aluminum or other metal having high thermal conduction. The cupis mounted on an insulating base or rim 4|. Wound about the outside ofthe cup 40 is a heating element 42- preferably insulated and bonded tothe cup 40 by a thin layer of thermoplastic material which is stable atnormal operating temperatures.

For the purpose of controlling the temperature of the crystal holders35, 36,. a thermostat is mounted in intimate conducting relation to theend surface of the cup 40 and means are provided within the cup forestablishing a path of direct heat flow from the thermostat to thecrystal holders. As shown in Fig. 3, the thermostat indicated as 43includes a bimetallic element 44 mounted in a metal enclosure 45 whichis soldered or otherwise bonded to the end surfaceof, the cup 40. Whilean adjustment 46 may be included if desired, the thermostats of the typeshown are commercially available and are normally supplied, sealed andcalibrated for a desired operating temperature.

Extending downwardly within the cup and intimately fastened .to the endsurface thereof for example by soldering, are fins 5| ,-52, 53.. Fins5|, 52 are spaced in inverted U-formation so as to closely embrace thecrystal holder 35 while fins 52, 53 embrace the crystal holder 35.Preferably fins 5I53 are laterally spaced from the wall of the cup 40defining an air space 54. Fur thermore the fins are made of highlyconductive metal and are substantially coextensive with the sides of thecrystal holders so that they constitute the primary thermal path betweenthe heating element 42 and the crystal holders. I

The connections from the heating coil and the thermostat are brought outto clips 51, 58, 59 (Fig. 4) which engage the contact posts 3'1, 38, 39projecting upwardly from the base assembly. The clips have suflicientfriction so that the heater assembly H is firmly, retained on the baseassembly III in spite of shock or vibration which may be encountered inuse.

The crystal ovens of the type disclosed herein have been found tocontrol the temperature and, therefore, frequency drift to a higherdegree of precision than comparable ovens commercially available. Forexample, in tests on the present oven and with the thermostat adjustedto maintain a. temperature 20 C. above normal ambient,

3 the ambient temperature has been suddenly varied by as much as 150 C.with less than 1 C. change in crystal temperature. Resistance of theunit to thermal shock of this sort has been made the subject ofinvestigation but is still not fully understood. It is believed to bedue to a combination of structural features. In the first place, it willbe noted that the crystal holders are surrounded almost completely by;the coil 42, and the cup 40 upon which it is wound serves as aneffective buffer against thermal shock.

Thus any change in ambient temperature cannot act directly on thecrystals butcan only be conducted to the crystals along the same thermalpath, namely the cup 40 and the fins attached thereto, which conductstheheat rrom'the coil. Furthermore, it will be noted that substantiallyall of the heat conducted to the crystals from the fins flows from theregion of the thermostat.

As 'a result any change in ambient conditions the walls of the crystalholders and covering almost their entire area, provide extremelyintimate coupling between the thermostat and the crystals. While it istrue that a certain amount of the heat from the coil 42 will pass to thecrystals through the air space 54 as a result or" radiation andconduction, the proportion of the heat transferred in this mannerappears small as compared to the amount of heat flowing up through thecup and downwardly through the fins.

The oven is protected by the cover I2 which protects the thermostat fromdrafts and serves as a buffer against thermal shock. It has been foundunnecessary to employ the thick layers of insulating materialconventionally used in the more accurate types of crystal ovens.

One of the advantages of the arrangement shown is that the heaterassembly ll may be readily pulled off and replaced by a similar unitwhen servicing becomes necessary. Similarly, the crystal holders 35, 36may be removed and holders having crystals of difierent frequencyquickly substituted. It will be apparent that the holder is simple andcompact, yet capable of being operated under conditions of. shock,vibration, and wide temperature swing encountered, for example, in theuse 01 aircraft and military radio apparatus.

We claim:

1. In an oven for a piezoelectric crystal the combination comprising abase having provision for mounting a crystal holder thereon with thecrystal holder extending upwardly away from metal mounted on said basefor enclosing said crystal holder, a thermostat thermally bonded to, theupper end of said cup, a heating coil I the base, an inverted cup ofthermally conductive wound on saidjcup and connected for controlby saidthermostat, and heat conducting fins fastened within said cup in theregion of saidthermostat and extending downwardly therein for embracingthe crystal. holder, said fins being spaced from the wall of said cup sothat heat.

is conducted to said crystal holder primarily from the region of saidthermostat.

2. In an oven for a piezoelectric crystal the combination comprising abase having provision for mounting a crystal holder thereon with thecrystal holder extending upwardly away from the base, an inverted cupmounted on said base for enclosing said crystal holder, said cup beingof thin walled construction and made of metal having good conductingproperties, a thermostat thermally bonded to the upper end of said cup,a heating coil wound on said cup and connected for control by saidthermostat, and a heat conducting fin fastened within said cup in theregion of said thermostat and extending downwardly therein in heatconducting relation to the crystal holder, the cup being so dimensionedas to provide an insulating space between the cup wall and the crystalholder so that heat is conducted to said holder primarily from theregion of said thermostat.

3. In. an oven for a piezoelectric crystal the combination comprising abase having provision for mounting two crystal holders of therectangular can type thereon with the holders extending upwardly away,from the base and separated slightly from one another, an inverted cupmounted on said base for enclosing said crystal holder, said cup beingof thin walled construction and made of metal having good conductingproperties, a thermostat thermally bonded to the end of said cup, aheating coil wound on said cup and connected for control by saidthermostat, and three heat conducting fins fastened within said cup inthe region of said thermostat and extending downwardly therein forstraddling the crystal holders, said fins being as dimensioned as to besubstantially coextensive with the wall of the adjacent holder and saidcup being so dimensioned as to provide an insulating space between thecup wall and the crystal holders.

4. In an oven for a piezoelectric crystal the combination comprising abase assembly and a heater assembly, said base assembly having a socketfor the reception of a crystal holder to maintain the same in uprightposition on the base assembly andhaving electrical connectors spacedabout the periphery thereof, said heater assembly comprising an invertedcup, having a heating coil wound about the wall of the cup and athermostat mounted on the upper end of the cup, said cup being of thinwalled construction and made 01 metal having good conducting properties,a rim of insulating material continuously engaging the lower edge of thecup for reinforcing the same and having separable electrical connectorsand connected to said coil and said thermostat and spaced to engage theconnectors on said base when said heater assembly is positioned thereon,and heat conducting fins thermally bonded to the end of said cup, andextending downwardly therein for embracing said crystal holder andsupplying heat thereto.

ERNEST E. OVERBEY.

JOHND. HOLMBECK.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,940,599 Morrison a. Dec. 19, 1933 1,967,184 Clapp r -I July17, 1934 2,438,345 Miller Mar. 23, 19418 2,470,134 Bitner May 17, 1949

